1. Field of the Invention
This invention is in the field of switching circuits and in particular the use of such circuits to change siren modes in emergency vehicle siren systems.
2. Brief Description of the Prior Art
Multivibrator circuits have been used for providing a variable voltage to a voltage controlled oscillator (VCO) in emergency vehicle siren circuits for many years. U.S. Pat. No. 3,747,092 entitled "ELECTRONIC SIREN CIRCUIT" inventor Gerald D. Smith, illustrates such a circuit. The VCO output frequency is amplified to provide a siren signal. In such a circuit, it is desirable to switch from one siren sound or mode to a distinctly different siren sound or mode for brief periods of time to provide an additional and more intensive warning sound during emergency situations. In previous circuits this was accomplished by manually holding a switch closed for the desired period of time; incorporating a predetermined time delay into a manual switch so that the second siren sound would be activated for the predetermined time upon the closing of a switch; or by manipulating a switch to activate the second siren sound and then manipulating a different switch to restore the first siren sound. The above approaches all have the disadvantages of inflexibility and the requirement of attention diverting manual operations during emergency vehicle operation.